Which institution used X-rays on NRRL 1951 to create the penicillin-producing strain X-1612?

The Carnegie Institution of Washington used X-rays on NRRL 1951 to create the penicillin-producing strain X-1612.

NRRL 1951 was an improved Penicillium strain selected during the American effort to increase penicillin production. Researchers at the Carnegie Institution of Washington irradiated it with X-rays, deliberately creating mutations and then screening the resulting moulds for higher output.

The resulting mutant, X-1612, produced about 300 milligrams of penicillin per litre of mould culture—twice the yield of NRRL 1951. This was an example of early mutation-based strain improvement: irradiation generated biological variation, while laboratory testing identified the useful producer.

The story continued at the University of Wisconsin, where researchers used ultraviolet radiation on X-1612 to produce strain Q-176. That distinction causes a frequent mix-up: Carnegie is linked to the X-ray step and X-1612, while Wisconsin is linked to the ultraviolet step and Q-176. Q-176 initially produced more penicillin than X-1612 but made a less desirable form until chemical feeding redirected production toward penicillin G.